All the versions have multiplayer, unless you got your copy through nefarious means. It should appear in the start menu, or if you go into the Outcast install folder there's a file named "jk2mp.exe". Also, the autorun menu has the option too.

I love Jedi Outcast, but I have it for Gamecube seeing how my PC would explode playing it. I thought Jedi Acedemy was the only online Jedi Knight game. Do a good amount of people play Outcast online, as soon as I get a new computer I plan on getting The Best of PC, and I wanna play it!